Whitby residents are being urged to conserve water "until further notice" after a ruptured water main was discovered at a Porirua reservoir.

Greater Wellington Regional Council spokesman Chris Laidlow said the crack was identified in a feeder main at Staithes Drive in Whitby this afternoon, in a feeder main coming from the Ascot Park reservoir.

The main is managed by the Porirua City Council.

Fixing the crack would take some time as the pipes were at least six metres underground, and repair teams would likely be working into the night, Mr Laidlow said.

He estimated that at 12.30pm the reservoirs at Whitby, Pukerua Bay, Plimmerton, Paremata and Ascot Park had been isolated and had enough water to last about 12 hours' average use.

He could not confirm whether this meant there would be issues with water supply, but asked residents to conserve water ''until further notice''.

About six months ago residents in Ascot Park, Aotea, Waitangirua and parts of Cannons Creek and Whitby were asked to reduce their water use as earthquake-proofing work was carried out on the city's water supply.